Method and apparatus for fabricating near spherical semiconductor single crystal particulate and the spherical product produced

ABSTRACT

An apparatus and a method for producing single crystal semiconductor particulate in near spherical shape and the particulate product so formed is accomplished by producing uniform, monosized, near spherical droplets; identifying the position of an undercooled droplet in a nucleation zone; and seeding the identified droplet in the nucleation zone to initiate single crystal growth in the droplet.

FIELD OF INVENTION

This invention relates to a method and apparatus for fabricating nearspherical single crystal particulate and to the spherical productproduced, and more particularly to such a method, apparatus and productmade of a semiconductor material such as silicon.

BACKGROUND OF INVENTION

Single crystal silicon spheres provide an ideal means of fabricatinghigh efficiency photovoltaic devices. These devices are described inU.S. Pat. Nos. 4,021,323 and 5,028,546. The desired size of such spheresis approximately 750 microns in diameter. Most recently technology isbeing developed to use single crystal silicon spheres as well as othersemiconductor materials as substrates in integrated circuits. Currentapplications envision the use of 1000 micron spheres, but ultimately awide range of sizes to accommodate different circuitry is likely. Inaddition, it is likely that such applications will utilize spheres ofother semiconductor materials, including germanium, gallium arsenide,and CuInGaSe₂.

Silicon, unlike conventional materials,is difficult to form into spheresby such techniques as gas atomization, rotary atomization or shotting.During solidification a volume expansion occurs that causes sphericalliquid silicon droplets to distort into highly irregular particulate.The irregular particulate that results requires extensive and expensivesubsequent processing as described in U.S. Pat. No. 4,430,150 to convertthe particles to a usable form. As a result other processes as describedin U.S. Pat. No. 4,637,855 for spheroidizing individual grains ofsilicon and U.S. Pat. Nos. 5,431,127 and 5,614,020 for fusing andspheroidizing measured amounts of loose silicon powder have beendeveloped. Some of these patents also describe how to heat siliconparticulate to melt it and then cool the silicon from the molten statesuch that single crystal particulate is formed.

All of these processes require multiple manufacturing steps in order toproduce single crystal silicon. It is well established in materialsmanufacturing that the more discrete steps there are in a manufacturingprocess the more costly the overall process. Thus all of the abovemanufacturing routes are inherently expensive as they first requireparticulate formation followed by additional special processes toconvert the particulate to single crystals.

Kirby in U.S. Pat. No. 4,322,379 describes a unique process forproducing near spherical, or tear drop shaped silicon particles that arealmost single crystals, "usually having less than 5 crystallites in an0.01 inch body" (250 microns) directly from the melt in a one stepprocess.

SUMMARY OF INVENTION

It is therefore an object of this invention to provide an improvedmethod and apparatus for making a near spherical single crystalsemiconductor particulate and the spherical product thereof.

It is a further object of this invention to provide such a method,apparatus and product in which the desired shape and single crystalstructure are formed in a single operation.

It is a further object of this invention to provide such a method andapparatus that are simpler, quicker, less expensive than prior art, andprovide a product with superior properties.

It is a further object of this invention to provide such a method,apparatus and product which can form solid or hollow microballoon,single crystal semiconductor spherical particulate.

The invention results from the realization that near spherical solidsingle and hollow, even more nearly spherical, single crystals ofadamantine semiconductors can be made directly from the melt in a singlestep process where solid or hollow droplets of molten material issue ina carefully controlled thermal environment. The thermal environment iscontrolled to produce a desirable level of undercooling in the dropwithout inducing crystallization. The droplet is cooled to anappropriate temperature without inducing crystallization and then seededto provide only one crystal nucleation site. Heterogeneous nucleationinduces solidification. By controlling the environment of the droplet,all of the liquid is consumed and a single crystal particulate isformed. The monosize droplet generator ensures that each droplet is thesame size and therefore can be processed in precisely the same manner.This identical processing of all the droplets is impossible withstochastic processes such as gas atomization that produce a wide rangeof droplet sizes. In the case of solid droplets, near-spherical teardropshaped single crystal particulate is formed; in the case of hollowdroplets or bubbles, the hollow single crystal particulate formed ismore nearly spherical.

This invention features an apparatus for producing single crystalparticulate in near spherical shape including a uniform dropletgeneration system for producing uniform monosized, near sphericaldroplets, and a nucleating system including a nucleation zone, a singlecrystal seeding device and a monitoring device for identifying theposition of a droplet in the nucleation zone and actuating the seedingdevice to initiate a single crystal growth in the droplet.

In a preferred embodiment the particulate may be silicon or anothersemiconductor. The nucleating device may include means for contacting adroplet with a single crystal particle of the same material as theparticulate. The means for contacting may include a probe or a particlelauncher. The monitoring device may include means for sensing thetemperature of a droplet in the nucleation zone. There may be heatingmeans responsive to the means for sensing the temperatures of a dropletfor maintaining the droplet at a predetermined undercooled temperaturerange in the nucleation zone. The heating means may be in the nucleationzone or in the uniform droplet generation system. There may be avelocity control system for controlling the velocity of the droplets inthe nucleation zone. It may include a blower system for providing acounterflowing stream of fluid in the nucleation zone. There may be amomentum dissipation zone for gradually reducing the velocity of thedroplets. It may include a curved passage. The curved passage maytransition from vertical to horizontal orientation. The momentumdissipation zone may also include a blower system for providing acounterflowing stream of fluid in a momentum dissipation zone. There maybe an annealing zone for relieving stress in the droplets. The annealingzone may include a heater and it may include a source of inert gas andisolation means. There may be a microballoon generator for formingdroplets with a hollow center. There may be a reactive gas employedfollowing the uniform droplet generator system; the reactive gas mayinclude oxygen or nitrogen. There may be a liquid metal atmospherefollowing the droplet generator system so that the droplets pass throughthe liquid metal prior to solidification; the liquid metal could begallium, indium, tin, antimony, zinc, copper or bismuth or an alloythereof. The microballoon generator may include means for injecting abubble of inert gas into each droplet formed in the uniform dropletproduction system, Droplets may be approximately 750 microns to 1250microns in diameter. They may be one millimeter in diameter. The uniformdroplet generator system may include a container for holding a liquidmelt, an aperture in a container for releasing the liquid, a vibratordevice for controlling formation of the droplets and a control devicefor sensing the size of the droplets released and operating the vibratordevice for maintaining the droplets at a uniform size. The particulatemay be doped silicon and the dopant may be Ga, In, Ge, Sn, As, Sb, Bi.

The invention also features a method for producing single crystalparticulate ill near spherical shape including producing uniformmonosized near spherical droplets; droplet in the nucleation zone toinitiate the single crystal growth in the droplet.

In a preferred embodiment the particulate may be a semiconductor such assilicon. Seeding may include contacting the droplet with a singlecrystal particle of the same material as the particulate. Contacting thedroplet may include engaging with the droplet a probe having a singlecrystal particle of the same material as the particulate. Contacting mayinclude launching at the droplet a single crystal particle of the samematerial as the particulate. The temperature of the droplet may besensed in nucleation zone and the temperature of the droplets may beadjusted to maintain a predetermined temperature range in that zone.Adjusting the temperature may include adjusting the heat applied to theforming droplet or to droplets in the nucleation zone, or both. Thevelocity of the droplets may be controlled in the nucleation zone. Thecontrol of the velocity may be done by introducing a counterflow fluidstream in the nucleation zone. The momentum of the droplets may begradually reduced in the momentum dissipation zone. The reduction of themomentum may include guiding a droplet through a curved path. Thatcurved path may change the droplet's direction from vertical tohorizontal. Reducing the momentum of the droplets may also includeintroducing a counterflowing fluid stream in the momentum dissipationzone. The droplets may be annealed to relieve stress. The annealing mayinclude heating in the absence of air. A bubble of inert gas may beintroduced into each droplet as it is produced. Droplets may be formedfrom approximately 750 microns to 1250 microns in diameter, for exampleone millimeter.

The invention also features a single crystal near spherical particulatemade by the method of the invention. The invention also features asingle crystal near spherical particulate which may be made of asemiconductor or silicon. The invention also features a single crystalnear spherical particulate having a near spherical shell and a hollowcore and may be made of a semiconductor or silicon.

DISCLOSURE OF PREFERRED EMBODIMENT

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

FIG. 1 is a simplified schematic block diagram of an apparatus forproducing single crystal particulate in near spherical shape accordingto this invention;

FIG. 2 is a more detailed schematic diagram of one implementation of theapparatus of FIG. 1 showing the droplet generation system, nucleationzone and momentum dissipation zone in greater detail;

FIG. 3 is a view similar to FIG. 1 showing an alternative implementationof the momentum dissipation zone and an annealing zone according to thisinvention;

FIG. 4 is an enlarged detailed view of a portion of the nucleation zoneof FIG. 2 with an alternative seeding device according to thisinvention;

FIG. 5 is an enlarged detailed view of the crucible and vibrator of FIG.2 with a side injector for creating hollow near spheres according tothis invention;

FIG. 6 is an enlarged detailed view of the crucible and vibrator of FIG.2 with a central injector for creating hollow near spheres according tothis invention;

FIG. 7 is an SEM micrograph of silicon solidified conventionally;

FIG. 8 is an SEM micrograph of a solid near sphere according to thisinvention;

FIG. 9 is an optical micrograph of a cross section of a solid nearsphere of silicon showing its single crystal structure;

FIG. 10 is an enlarged illustrative view of a hollow silicon spherehaving a refractory oxide coating; and

FIG. 11 is a flow chart of the method according to this invention.

Semiconductor materials in general are difficult to form into spheres bysuch techniques as gas atomization, rotary atomization or shotting.Indeed, liquid germanium droplets are known to distort into highlyirregular shapes upon solidification. [See, for instance, R. F.Cochrane, P. V. Evans, and A. L. Greer, "Containerless Solidification ofAlloys in a Drop-tube,", Mat. Sci. Eng. 98, 99-103 (1988).] It isbelieved that silicon and germanium exhibit similar macroscopic andmicroscopic solidification behavior because both materials areclose-packed metallic liquids and tetrahedrally-bonded semiconductingsolids with an adamantine (diamond-like) crystal structure. Thus, bothmaterials exhibit a volume expansion upon solidification, high entropiesof fusion, and similar mechanisms of crystal growth on a microscopiclevel. Other semiconductor materials, such as the so-called "III-Vcompounds" (GaAs, GaSb, InSb, InP, InAs), also exhibit a volumeexpansion upon solidification of a similar magnitude, a transition froma close-packed metallic liquid state to an adamantine semiconductingsolid-state, and mechanisms of crystal growth similar to those of Ge andSi. [See O. Madelung, M. Schultz, H. Weiss, Eds. "Semiconductors:Technology of III-V, II-VI and Non-tetrahedrally Bonded Compounds, ",Landold-Bornstein, vol. 17d, p. 27 (Springer-Verlag, New York, 1984)].Thus, although the technology disclosed herein is discussed mostspecifically with reference to silicon, it is clear that it can beapplied, with suitable minor modifications, to any material composedprimarily of any of the tetrahedrally-bonded semiconductor substances,including binary, ternary, quartenary (including CuInGaSe₂), and highercompounds. Herein semiconductor means adamantine semiconductors in pureand doped form. Dopants may include, e.g., B, Al, Ga, In, N, P, As, Sb,Se and Ge. Certain dopants may be added to improve sphericity, such asGa, In, Ge, In, As, Sb and Bi, for example. A partial listing of themany semiconductor materials to which this invention applies may befound in the CRC Handbook of Chemistry and Physics, 71 st Ed., David R.Lide ed, pages 1254-1256.

There is shown in FIG. 1 an apparatus for producing single crystalparticulate 10 in near spherical shape including a uniform dropletgeneration system 12 having a vibration shaft and droplet orifice asdescribed by Professor Jung-Hoon Chun of M.I.T. in U.S. Pat. No.5,266,098, Prof. Chun also describes measurement and feedback controlsto provide precise droplet size and production rates. The frequency ofdroplet production must be carefully controlled to prevent dropletcollisions especially as the droplets pass through this nucleation zone.

A standard quartz crucible may be used for melting the semiconductor orother material such as silicon, and a continuous feed device may be usedto constantly supply the crucible. At any one time only a very smallamount of silicon is molten in the crucible, thus providing a shortresidence time of the molten silicon in the crucible to minimize oxygenpickup by the silicon. For example, as little as 40 grams can be moltenand using a droplet production rate of 4 grams per minute would ensurethat the mean contact time of the silicon with the crucible was only onthe order of ten minutes. This would compare very favorably with theCzochralski crystal growth in which silicon processed in batches mustreside in contact with the crucible for periods of an hour or more. Useof even smaller molten masses will further reduce the contact time. Ifeven higher purity is desired the continuous feed system could becoupled with cold hearth copper crucible induction melt systemsinitially developed for the growth of very pure cubic zirconia and nowbeing applied to such reactive materials as titanium.

The uniform droplet generation system 12 sits on top of a drop tower 14which also includes a nucleation system 16, momentum dissipation zone 18and collection zone 20. The nucleation system 16 below the uniformdroplet droplet generation system 12 is used to control the thermalenvironment of the droplets 22. The nucleation system is surrounded byheaters and contains an inert gas such as He, Ar, Kr, or Xe, or reactivegas such as air, O₂, N₂ ; carbonaceous, borane, silane, germane,pnictide, chalcogen, halogen gases; or any mixture of inert and reactivegases, especially mixtures of this type typically used for chemicalvapor deposition, ion implantation, epitaxial growth, or diffusioncoating of semiconductor substrates. Rather than a reactive or an inertgas, the nucleation system could contain a reactive or inert liquidmetal including gallium, indium, tin, antimony, bismuth, zinc, copper,or any other suitable liquid metal, or any alloy composed of suchmetals, including the binary, ternary, and higher eutectics of galliumand indium. If a liquid metal is employed as the medium in whichnucleation is achieved, the preferred embodiment is similar to theapparatus shown in FIG. 1 if the density of the semiconductor is greaterthan the liquid metal but will be upside down if the density of thesemiconductor is less than the liquid metal. The counterflowing gasvelocity and pressure are sufficient to slow the passage of the dropletthrough the nucleation system so the droplet vibration is substantiallydamped before the crystal is nucleated in the droplet 22 which typicallyis in the range of 100μ-1250μ. Nucleation may be accomplished by anumber of means such as pricking the droplet with a silicon seed on theend of a probe or showering or spraying the droplets 22 with a seed asthey pass by. Seeding the droplets requires precise timing involvingvision systems and pyrometry as shown. To ensure that the droplet is inthe desired undercooling prior to being seeded. For example, again usingthe example of four grams per minute of silicon, the desiredundercooling where the droplet is seeded is 0-50° C. Once seeded, thegrowth time for a one millimeter droplet single crystal is on the orderof one minute. The droplet must be completely solidified before leavingthe nucleation zone. The actual time required by the droplet isdependent on droplet size, gas temperature, gas velocity, gas pressureand gas type. The values for the above operating parameters are easilydetermined by those skilled in the art by the use of Stokes Settling Lawand the equation governing heat loss from a small sphere in a convectiveenvironment. The droplet motion must be precisely controlled to insurecomplete solidification and to avoid collision with other droplets.Static gas would work but would require extended drop) lengths.

An annealing and momentum conversion zone may be provided at the bottomof the chamber. Semiconductor materials have a tendency to fracture dueto a sensitivity to mechanically and thermally induced stresses. One wayof absorbing the momentum of the falling particle to prevent cracking isto simply slowly convert the direction of motion from vertical tohorizontal via the use of a curved section of pipe. Once the motion ofthe solidified particle is horizontal it may be allowed to come to restor continue to be transported slowly in an annealing zone. The annealingzone may be maintained at about 1380° C. and the particles will annealwithin a few minutes. Temperatures as low as 1200° C. may be employed,but at the cost of increased annealing times of up to 3-5 hours and evenlonger. If a vacuum anneal is desired the annealing zone may be isolatedby adding a gate valve and evacuating it with a conventional vacuumpump. The uniform droplet generation system 12 may include means toproduce bubbles as described in U.S. Pat. No. 4,582,534. In this casemolten silicon bubbles are produced rather than droplets. Undercoolingnucleation and growth of single crystal hollow spheres is accomplishedwith the same means as the droplets, but because the droplets are hollowthe solidifying silicon has room to expand internally and the dropletswill retain a more nearly spherical shape after solidification.

The uniform droplet generation system 12, FIG. 2, may include a standardcrucible 30 in which silicon or some other suitable material such as asemiconductor is melted. The droplets 22 forming at orifice 32 issueunder control of the vibrator 34 driven by amplifier 36 which is in turncontrolled by microprocessor 38 as taught by Chun, supra. A strobe lamp40 driven by microprocessor 38 illuminates droplets 22 as they pass infront of CCD camera 42. An analysis by microprocessor 38 of the outputfrom camera 42 indicates whether the droplets are too large, too small,or properly sized. If they are too large, the signal to vibrator 34 isadjusted to increase the vibration frequency; if the droplets are toosmall, the vibration frequency is decreased. As the droplets 22 continueto fall they enter the nucleation system 16 where they are individuallyidentified and their temperature sensed by a two-dimensional imagingpyrometer 44. If the position of the particular droplet identified isconfirmed by imaging system 46, pulse generator 48 produces a pulse tooperate solenoid 50 and drive nucleating needle 52 to seed that droplet.Heterogeneous nucleation may be stimulated by other means such asvibration, a pressure pulse or even a laser pulse. If the temperature ofthe droplets is not at the proper undercooled temperature fornucleating, then heater control 54 may be operated to increase ordecrease the heating of heater 56 or heater control 58 may do likewisewith heater 60 in uniform droplet generation system 12. The apparatusmay employ either one or both of the heaters and heater controls forthis purpose. The temperature of the droplets may also be controlled byhaving them move more slowly or spend more dwell time within thenucleation zone 16. This can be accomplished by using a blower system 60wherein a blower motor 62 feeds a gas, typically an inert gas such asthose mentioned above, through input channel 64 to outlet: 66 to providea counterflow of the gas upwardly as indicated by arrow 68 against theflow of droplets 22. The gas is collected at outlet 70 and returnedthrough channel 72 to blower motor 62. A heat exchanger 63 may be usedto control the temperature of the gas being used to create thecounterflow stream at 68. The momentum may be dissipated or arrestedusing a blower system 80 similar to blower system 60 shown in thenucleation zone and would have the same parts and could also use a heatexchanger, all being referred to with the same reference numerals as forblower system 60 but accompanied by a lower case letter a.

Although the apparatus 10 including the cooling tower 14 has beendepicted in FIG. 2 as being upright and vertical, this is not anecessary limitation of the invention as the entire system as shown inFIG. 2 could be turned upside down or could be operated on its side orat an inclination, with suitable accommodation being made. Additionallythe tower 14 is shown for schematic purposes as being much larger indiameter than the droplets. To incur an aerodynamic centering the towerdiameter could be within 11/2 droplet diameters.

In an alternative construction, momentum dissipation zone 18a, FIG. 3,may rely on a change of direction from the vertical to the horizontal byusing a curved pipe 90 that slowly and gently changes the direction ofthe falling stream of droplets 22 as they strike its lower side 92. Inaddition, a blower system 80a similar in all respects to blower system80, FIG. 2, may be employed. Also shown in FIG. 3 is the cooling andcollection zone 20a incorporating an annealing zone 100 fed by gatevalve 102 and having a chamber 104 suitably heated by heaters 106 toprovide the proper annealing temperatures for the proper period of time.For example, silicon can be annealed using temperatures ranging fromjust below the melting point (1420° C.) down to about 1200° C. At veryhigh temperatures annealing is accomplished within a few minutes, at1200° C. annealing can take up to 5 hours. It is noted the momentumconversion zone is preferably not a cold zone. The particles are allowedto substantially cool below 1200° C., if at all, only after they haveessentially stopped moving. Furthermore, a source 108 of pressurizedinert gas, argon or helium, for example, can be employed in conjunctionwith a vacuum pump 110 for evacuating chamber 104 and filling it withthe selected/inert or reactive gas.

Although in FIG. 2 a solenoid 50 with needle 52 was indicated as themeans for introducing the nucleation seed to the droplet, this is not anecessary limitation of the invention. For example, as shown in FIG. 4,nucleation can be induced by a seed launched from a pneumatic tube, apressure pulse from a piezo electric crystal, a beam from a laser, or avibration from a sonic source, all of which are represented at 121, FIG.4.

If it is desired to have hollow or balloon spheres instead of solidspheres, this can be accomplished using a modified form of the hollowsphere or microballoon generator as described by Torobinin in U.S. Pat.No. 4,582,534. Such a device is shown in FIG. 5, using a crucible 30awhich contains the melt such as silicon melt 122 and has positioned init a vibrator 34a such as a piezoelectric transducer or wound electricalsolenoid which drives a refractory plunger 124. In the nozzle 126 belowthe plunger 124 there may be inserted an inert gas injector 128 thatinjects a bubble of inert gas 130 into the droplets as they are formingso that when they form they form hollow or balloon shaped droplets 22athat have a very near spherical shell 132 with a hollow core 134. Theinjector may also be provided centrally down the middle of refractoryplunger 124a, FIG. 6.

FIG. 7 shows the irregular particulate that results when silicon issolidified conventionally. The resultant silicon solid near spheresshown in FIG. 8 are made in accordance with this invention. The crosssection shown in FIG. 9 has a featureless microstructure characteristicof single crystals. By using the hollow droplet approach with controllednucleation and solidification the distortion of the droplet shown inFIG. 7 can be avoided completely and due to surface tension of theliquid an outer spherical shell even more spherical than shown in FIG. 8will result. By oxidizing the still liquid shell a hard solid sphericalcoating is provided and as the rest of the liquid solidifies it canexpand inwardly and partially fill the internal hollow pore withoutcausing a rupture or distortion of the spherical shell as shownschematically in FIG. 10. This oxide shell can be created by using argoncontaining 5% oxygen in the droplet generation and nucleation zones. 1%to 10% mixtures have been shown to work equally well. Additionally,nitride coatings have been produced using nitrogen and it is thoughtcarbide coatings can be produced using carbonaceous gases.

The method according to this invention begins with melting the siliconor other semiconductor material and discharging the droplets, step 200,FIG. 11, and then sensing the droplet size 202. If it is desired to havethe hollow or balloon type of droplet instead of the solid droplet thereis injected an inert gas bubble in step 202. After the droplet isdischarged the droplet size is sensed in step 204. If it is unacceptablethen the droplet size is adjusted and the system returns to step 200. Ifthe droplet is an acceptable size, then the system senses the droplettemperature in step 208. If the temperature is unacceptable thetemperature is adjusted in step 210 and the system returns to step 200.If it is acceptable, the seeding system is actuated in step 210. Afterthis the momentum is slowly dissipated in step 212 which may beaccomplished as indicated for example by counterflows and/or redirectionof the falling droplets. Finally, the near spheres are cooled andcollected in step 214 and they may be annealed in a vacuum or in aninert gas or reactive gas atmosphere. Although the disclosed system isdescribed using silicon or germanium, any suitable material may be used.Suitable materials include doped or pure semiconducting materialconsisting primarily of silicon, germanium, or any of the othertetrahedrally-bonded (adamantine) semiconductors, which include binarycompounds such as the "III-V" compounds (GaAs, GaSb, InSb, InP, InAs),ternary compounds, quaternary compounds (such as CuInGaSe₂), and highercompounds.

Although specific features of this invention are shown in some drawingsand not others, this is for convenience only as each feature may becombined with any or all of the other features in accordance with theinvention.

Other embodiments will occur to those skilled in the art and are withinthe following claims:

What is claimed is:
 1. A method for producing single crystalsemiconductor particulate in near spherical shape comprising:producinguniform, monosized near spherical droplets; identifying the position ofa droplet in a nucleation zone; and seeding the identified droplet inthe nucleation zone to initiate single crystal growth in the droplet. 2.The method of claim 1 in which said particulate is a semiconductor. 3.The method of claim 2 in which said particulate is silicon.
 4. Themethod of claim 1 in which said seeding includes contacting the dropletwith a single crystal particle of the same material as the particulate.5. The method of claim 4 in which said contacting includes engaging withthe droplet a probe having a single crystal particle of the samematerial as the particulate.
 6. The method of claim 4 in which saidcontacting includes launching at the droplet a single crystal particleof the same material as the particulate.
 7. The method of claim 1further including sensing the temperature of a droplet in the nucleationzone and adjusting the temperature of the droplets to maintain apredetermined temperature range.
 8. The method of claim 7 in whichadjusting the temperature includes adjusting the heat applied to theforming droplets.
 9. The method of claim 7 in which adjusting thetemperature includes adjusting the heat applied to the droplets in thenucleation zone.
 10. The method of claim 1 further including controllingthe velocity of the droplets in the nucleation zone.
 11. The method ofclaim 10 in which controlling the velocity includes introducing acounterflow fluid stream in said nucleation zone.
 12. The method ofclaim 1 further including gradually reducing the momentum of thedroplets in a momentum dissipation zone.
 13. The method of claim 12 inwhich reducing the momentum includes guiding the droplets through acurved path.
 14. The method of claim 13 in which the curved path changesthe droplets direction from vertical to horizontal.
 15. The method ofclaim 12 in which reducing the momentum of said droplets includesintroducing a counterflowing fluid steam in said momentum dissipationzone.
 16. The method of claim 1 further including annealing the dropletsto relieve stress.
 17. The method of claim 16 in which annealingincludes heating the droplets in the absence of air.
 18. The method ofclaim 1 further including injecting a bubble of inert gas into eachdroplet as it is produced.
 19. The method of claim 1 in which saiddroplets are formed approximately 750 μm to 1250 μm in diameter.
 20. Themethod of claim 1 in which said droplets are formed 1000 μm in diameter.